Many parts of the world lack sufficient rainfall at different times of the year to maintain the health of turf and landscaping. Irrigation systems are therefore used to deliver water to such vegetation from municipal water supplies and wells according to a watering schedule. A typical irrigation system comprises a programmable electronic controller that turns valves ON and OFF to deliver water through a plurality of sprinklers connected to the valves via subterranean pipes. These sprinklers are usually rotor-type, impact, spray or rotary-stream sprinklers. Pressure regulators have been installed in residential and commercial irrigation systems externally of the sprinklers. U.S. Pat. No. 5,257,646 of Meyer discloses an in-line pressure regulator for an irrigation system. Pressure regulators have also been incorporated into the sprinklers themselves. U.S. Pat. No. 5,779,148 of Saarem et al. discloses a spray sprinkler with a pressure regulator in its extensible riser. Published U.S. Patent Application No. 2007/0007364 of Gregory discloses a rotor-type sprinkler with a pressure regulator located at the lower end of the riser below the turbine.